Embryos of many plant seeds accumulate sucrose and the raffinose family of oligosaccharides (RSO), such as raffinose, stachyose and verbascose, as the major soluble sugars in mature seeds (Horbowicz et al., Seed Sci. Res. 4:385-405 (1994); Obendorf, See Sci. Res. 7:63-74 (1997)). Soybean (Glycine max (L.) Merrill) seeds accumulate soluble carbohydrates, primarily sucrose, raffinose, and stachyose and lesser amounts of galactopinitol A, galactopinitol B, ciceritol, and fagopyritol B1in axis and cotyledon tissues as part of the seed maturation process (Obendorf et al., Crop Science 38:78-84 (1998)). By contrast, embryos of maturing buckwheat (Fagopyrum esculentum Moench) seeds accumulate fagopyritols, galactosyl derivatives of D-chiro-inositol, instead of raffinose and stachyose (Horbowicz et al., Planta 205:1-11 (1998)). Six fagopyritols, in two different series, are present in buckwheat embryos: fagopyritol A1 (α-D-galactopyranosyl-(1→3)-1D-chiro-inositol), fagopyritol A2 (α-D-galactopyranosyl-(1→6)-α-D-galactopyranosyl-(1→3)-1D-chiro-inositol), fagopyritol A3 (α-D-galactopyranosyl-(1→6)-α-D-galactopyranosyl-(1→6)-α-D-galactopyranosyl-(1→3)-1D-chiro-inositol), fagopyritol B1 (α-D-galactopyranosyl-(1→2)-1D-chiro-inositol), fagopyritol B2 (α-D-galactopyranosyl-(1→6)-α-D-galactopyranosyl-(1→2)-1D-chiro-inositol), and fagopyritol B3 (α-D-galactopyranosyl-(1→6)-α-D-galactopyranosyl-(1→6)-α-D-galactopyranosyl-(1→2)-1D-chiro-inositol) (Horbowicz et al., Planta 205:1-11 (1998); Szczecinski et al., Bull. Polish Acad. Sci., Chem. 46:9-13 (1998); Obendorf et al., Carbohydr. Res. 328:623-627 (2000); Steadman et al., Carbohydr. Res. 331:19-25 (2001)). Fagopyritols are concentrated in the axis and cotyledon tissues of embryos in mature buckwheat seeds (Horbowicz et al., Planta 205:1-11 (1998)). Buckwheat bran, a commercial milling fraction (Steadman et al., J. Cereal Sci. 33:271-278 (2001)), is a rich source of fagopyritols (Steadman et al., J. Agric. Food Chem. 48:2843-2847 (2000)).
Fagopyritols are of considerable interest for the treatment of non-insulin dependent diabetes mellitus (NIDDM) and polycystic ovary syndrome (PCOS), both insulin response disorders. Fagopyritol A1 is isosteric with 2-amino-2-deoxy-α-D-galactopyranosyl-(1→3)-1D-chiro-inositol (Berlin et al., Tetrahedron Lett. 31:1109-1112 (1990)) related to a putative insulin mediator (Berlin et al., Tetrahedron Lett. 31:1109-1112 (1990); Lamer et al., Biochem. Biophys. Res. Comm. 151:1416-1426 (1988)) deficient in subjects with NIDDM (Fonteles et al., Diabetologia 39:731-734 (1996); Lamer et al., Diabetes Rev. 7:217-231 (1999)) and PCOS (Nestler et al., J. Clin. Endocrin. Metab. 83:2001-2005 (1998); Nestler et al., New England J. Med. 340:1314-1320 (1999); Nestler et al., J. Pediatric Endocrin. Metab. 13 (Suppl. 5):1295-1298 (2000)).
Enzymes (fagopyritol synthases) catalyzing the biosynthesis of fagopyritols in buckwheat or other plants have not been described. The present invention is directed to overcoming this and other deficiencies in the prior art.